Powder metallurgical processes are well known techniques for producing metal articles in forms that otherwise are difficult to manufacture. Moreover, by selectively blending the alloying materials before the thermomechanical processing ("TMP") steps are undertaken, the physical and chemical characteristics of the ultimate alloy can be controlled.
Of the various methods for manufacturing shaped articles, the canning process is the most common. Briefly, the metallic powders (elemental or pre-alloyed) are introduced into a mild steel can which is sealed under vacuum or in an non-oxidizing atmosphere. The can is then hot worked to form a near net shape. The can is mechanically or chemically removed.
The difficulty here is that the use of a can is involved and requires additional steps and expense. The disadvantages of the can are: (1) the cost of manufacturing the can, (2) the process of adding the powder to the can and evacuating it (or otherwise treating it) to prevent the powder from oxidizing during subsequent heating steps, and (3) the removal of the can (the decanning operation) from the product.
Powder metallurgy techniques frequently involve hot working as a means for bringing consolidated metallic bodies to near hundred percent density. As stated beforehand, hot working and heating of powders must be conducted in a non-oxidizing atmosphere to prevent oxidation. Oxidation must be avoided since it will limit the density of the final product and, simultaneously, deleteriously affect its properties. Due to the relatively large surface area of the individual particles and the tortuous paths therebetween, powders are easily prone to debilitating oxidation. Accordingly, the powder is placed in a can (or if in a hot isostatic press-an elastic bladder) and treated.
Gas atomized powders compound the problem even further since they are clean (that is, devoid of impurities that, in conventional powders, act as "glue") and are generally spherical in shape. These powders are not cold compactable and hot compaction processes add appreciably to product cost. Spheres do not compact well since there are no irregular surface occlusions (as in conventional powders) to grab and lock onto.
It is desirable to develop a method to produce a billet made from gas atomized powders that may be extruded without the use of a can while simultaneously eliminating the problems associated with oxidation.
Representative references relating to the instant art include: U.S. Pat. No. 3,549,357 in which iron and iron-base alloys are tumbled with a number of elements to coat a sintered object; U.S. Pat. No. 3,798,740 in which a consolidated metal powder is coated with glass prior to extrusion; and U.S. Pat. No. 3,740,215 in which consolidated metal powders are surface sealed and oxidized prior to extrusion.